Lava flows are the most common of the direct volcanic
hazards in Hawai'i. Flows may endanger people's property, livelihood,
and peace of mind, but seldom their lives. The fronts of Hawai'ian
lava flows generally move more slowly than the speed at which people
walk, although the lava in the channel behind the front may be flowing
much faster. On steep slopes a large flow could travel rapidly enough
to endanger persons in its path. During the 1950 eruption of Mauna
Loa, a flow front advanced at an average speed of almost 6 mph for
over 2 hours.

The speed of a lava flow is determined not only by
the steepness of the terrain, but also by the volume of lava that
is erupted, because large flows tend to advance more rapidly than
do small flows. The distance that a flow travels ultimately depends
both on the eruption rate and on the duration of the eruption.

The chemical composition of lava will also affect
how rapidly a lava flows. Most Hawai'ian lavas are classified as basalts,
but within this category there are many types. Some basalts are more
fluid and will flow at greater speeds than others. The eruption of
Hualalai in 1800-1801 produced lava flows that appear to have been
more fluid than flows from similar eruptions on Kilauea and Mauna
Loa.

The continuing eruption on Kilauea's east
rift zone, which began in 1983, provides a very
good example of two common, but very different, types of eruptive
behaviour; rapidly-moving flows produced during brief, high-volume
eruptions, and slow-moving flows created by a prolonged low-volume
eruption. The Pu'u 'O'o-Kupaianaha eruption on Kilauea's east
rift
zone is the example. The episodic eruptions at the vent, which
was active from 1983 through June 1986, produced a large volume
of lava within a few hours' time. These outbursts were characterised
by spectacular lava fountains and lava flows that moved rapidly
down the volcano's southern flank. The flows entered the Royal
Gardens subdivision during 7 episodes and destroyed 16 homes.
Each flow was short-lived, however, and stagnated soon after
the lava fountains died. None of these flows reached the coastline.

In July 1986, the site of the eruption shifted
to the Kupaianaha vent, 1.8 miles to the Northeast of Pu'u
'O'o. Kupaianaha erupted almost continuously for over 5 years
but at a much slower rate than Pu'u 'O'o. During the first
few months of activity at Kupaianaha, the lava flows did
not advance more than a mile beyond the vent. But after months
of continuous eruption, a lava tube system formed as channelled
lava flows gradually formed roofs, enclosing the rivers of
lava within. Lava tubes are significant in terms of hazard,
since the tubes insulate the lava and allow it to flow much
farther before cooling and stopping.

The hazards posed by a prolonged low-volume
eruption became apparent as lava tubes from Kupaianaha extended
toward the Kalapana coast. From November 1986 to October 1991,
tube-fed flows repeatedly engulfed residential areas on the
coastal plain, destroying 165 houses. Although these flows
buried many acres within a single day, there was time to
evacuate residents. Warnings issued by the Hawai'i County
Civil Defence allowed people enough time to remove most of
their belongings and, in some cases, even to dismantle and
move their homes. In 1992, the threat to inhabited areas eased
when the eruption shifted to new vents on the Southwestflank
of the Pu'u 'O'o cone, inside Hawai'i Volcanoes
National Park.

A
Kalapana house is ignited by a lava flow.
By 1991, this community lay buried beneath 50 - 75 feet of
lava.

The chief threat of lava flows to property owners
is that the flows may burn structures and bury land. There are other
effects, however, that may be almost as disruptive, as the Kalapana
community discovered during the repeated inundations of the area by
lava. In addition to destroying homes, the flows covered almost 2
miles of coastal highway. Some residents were forced to move when
the highway closures increased their daily commute by nearly 100 miles.
Many more residents in the Kalapana area were faced with financial
losses as land values
dropped and insurance companies refused to issue new homeowners policies.

Even houses that are spared by the lava, however,
may be rendered uninhabitable when the roads and utility lines leading
to them are destroyed. By 1996, lava flows from Kilauea's eruption
had covered 8 miles of the coastal highway, isolating the few structures
that remained within the area.

To simplify land-use planning, the HVO have created
a hazard map of Hawai'i to illustrate the
lava hazard.

In
the Hawai'ian hazard map, zone '1' is the most hazardous, '9' the
least.
Zone '1' is actually the south flanks and rift zones of the Hawai'ian
volcanoes.